Liquid crystal display having uniform interval between pixel electrodes

ABSTRACT

A liquid crystal display, and a fabricating method thereof, wherein pixel electrodes are highly accurately located relative to opaque elements, such as gate lines, data lines, or auxilarly lines, beneficially by using opaque elements as masking elements when exposing a photosensitive layer through a substrate. The angle of the irradiating light through the substrate can be changed to achieve a desired relative location. A pixel electrode can be located within 1 μm of a data line. Image stain defects can be reduced.

This application is a Divisional of prior U.S. application Ser. No.09/725,154, filed Nov. 29, 2000, now U.S. Pat. No. 6,778,250 and claimsthe benefit of Korean Patent Application No. 1999-18903, filed on May25, 1999, which is hereby incorporated by reference for all purposes asif fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to liquid crystal displays. Moreparticularly it relates to active matrix liquid crystal displays (AMLCD)having well-aligned pixel electodes, and to a method of fabricating suchactive matrix liquid crystal displays.

2. Discussion of the Related Art

An active matrix liquid crystal display is typically fabricated byjoining an upper plate to a lower plate, and then injecting a liquidcrystal material between the plates. A lower plate usually includes aplurality of pixel cells that are formed from switching devices (usuallythin film transistors) and pixel electrodes. Such lower plates furtherinclude a plurality of drive lines that connect drive signals to thepixel cells. An upper plate usually includes a plurality of colorfilters and a common electrode. To complete an active matrix liquidcrystal display, polarizing plates are attached to the upper and lowerpirates.

FIG. 1 schematically illustrates a typical prior art active matrixliquid crystal display. As illustrated, data lines, including a dataline 15L, cross a plurality of gate lines, including gate lines 11L and10L. The areas between the data lines and the gate lines define pixelcell regions. A thin film transistor (hereinafter abbreviated TFT) thatacts as a switching device is formed at intersections between the datalines and the gate lines.

A TFT includes a gate electrode 11G, which is a protrusion from the gateline 11L, a source electrode 15S, which is a protrusion from the dataline 15L, a drain electrode 15D, and an active layer 13. The activelayer is overlapped by the electrodes. As shown, a pixel electrode 17connects to the drain electrode 15D.

Prior art active matrix liquid crystal displays are usually fabricatedusing photolithograpy. For example, to form the data line 15L and sourceelectrode 15S, the gate line 11L and gate electrode 11G, and the drainelectrode 15D a metallic layer is deposited on a prepared substrate. Thedeposited metallic layer is then coated with a photoresist layer. Thedeposited metallic layer is then patterned by selectivly exposing thephotoresist layer through a prepared mask using a light source that isabove the metallic layer. The exposed photoresist layer is then etchedto leave metallic conductors for the lines and electrodes. Pixelelectrodes are then formed in the same manner. However, pixel electrodesare typically fabricated after the lines and electrodes. Significantly,the pixel electrodes are fabricated from a transparent material.

While the photolithographic process described above has proven useful,it has problems. One particular problem when fabricating prior artactive matrix liquid crystal displays is the likelyhood of misalignmentof the pixel electrodes relative to other features. Such misalignmentmay be caused by misalignment of exposure masks or of the exposureapparatus, or by an etch deviation due to etch conditions.

FIG. 2 assists the understanding of pixel electrode misalignment byshowing a cross-sectional view taken along the line I-I′ of FIG. 1.Initially, a gate insulating layer 12 is formed on a substrate 100. Thedata line 15L is then photolithographically formed on the gateinsulating layer 12. A protection layer 16 is then formed over thestructure. Pixel electrodes 17 are then photolithographically formed onthe protection layer 16. Ideally, the data line 15L is centered betweenthe pixel electrodes such that the intervals L and R are the same.Unfortunately, the locations of the pixel electrodes can deviate fromtheir intended locations. Such deviations can be caused divisionalexposure.

With divisional exposure, each exposure step requires new exposureequipment, such as a photomask, to be set-up. Thus, it is very difficultto control the intervals L and R such that they are even. As a result,image defects referred to as image stains are created. Furthermore,cross-talk between the pixel electrodes and the data lines becomes moresevere due to deviations of parasitic capacitances.

Therefore, an improved active matrix liquid crystal display, and a newmethod of fabricating such an active matrix liquid crystal display,having accurately positioned pixel electrodes would be beneficial.

SUMMARY OF THE INVENTION

Accordingly, the principles of the present invention are directed to aliquid crystal display and to a fabricating method thereof thatsubstantially obviate one or more of the problems due to limitations anddisadvantages of the related art.

An object of the present invention is to provide a liquid crystaldisplay, and a fabricating method thereof, which has accuratelypositioned pixel electrodes. Uniform intervals between pixel electrodesand data lines are created by patterning the pixel electrodes using aself-alignment technique by exposing the pixel electrodes through asubstrate.

Additional features and advantages of the invention will be set forth inthe description which follows and in part will be apparent from thedescription, or may be learned by practice of the invention. Theobjectives and other advantages of the invention will be realized andattained by the structure particularly pointed out in the writtendescription and claims hereof as well as the appended drawings.

To achieve these and other advantages and in accordance with the purposeof the present invention, as embodied and broadly described, the presentinvention includes a liquid crystal display, wherein a plurality of gatelines cross a plurality of data lines to define locations of a pluralityof pixel cells. Switching devices are formed at intersections of gatelines and data lines. Pixel electrodes are formed at the pixel cells,and each pixel electrode connects to a switching device. The pixelelectrodes are beneficially formed such that the distance between eachside of a pixel electrode and a side of a data line adjacent to thepixel electrode is accurately controlled, preferrably less than or equalto 1 μm.

In another aspect, the present invention includes the steps of providinga substrate, fabricating a plurality of gate lines and a plurality ofcrossing data lines that define a plurality of pixel cells, and formingswitching devices at intersections of the gate lines and the data lines.The present invention further includes the steps of depositing aprotection layer over the switching devices, gate lines, data lines, andsubstrate, forming contact holes through the protection layer to exposeelectrodes of the switching devices, and forming a transparentconductive layer over the exposed surface of the substrate, includingthe exposed electrodes. Additional steps include forming a negative typephotoresist layer on the transparent conductive layer, selectivelyexposing the negative type photoresist layer through the substrate suchthat the data lines act as masks, forming a photoresist pattern bydeveloping the selectively-exposed photoresist layer, and etching thetransparent conductive layer. Beneficially, before developing thenegative type photoresist layer an exposing source and a mask that areabove the photoresist layer can also be used to expose the transparentconductive layer.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWING

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention.

In the drawings:

FIG. 1 is shows a simplified schematic view of a prior art LCD;

FIG. 2 is a cross-sectional view along the line I-I′ in FIG. 1;

FIG. 3 shows a simplified schematic view of an LCD according to anembodiment of the present invention;

FIG. 4 is a cross-sectional view along the line II-II′ in FIG. 3;

FIGS. 5A to 5F show cross-sectional views of the LCD shown in FIG. 3during its fabrication;

FIGS. 6A and 6B are cross-sectional views showing how an intervalbetween a pixel electrode and a data line can be controlled by using thedirection of an exposing light; and

FIG. 7 is a cross-sectional view of an LCD that presents the intervalrange between a pixel electrode and a data line according to the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the preferred embodiment of thepresent invention, example of which is illustrated in the accompanyingdrawings.

FIG. 3 shows a simplified schematic view of an LCD according to anembodiment of the present invention, while FIG. 4 shows across-sectional view of that embodiment taken along the line II-II′ ofFIG. 3. The illustrated embodiment has a structure in which a pluralityof gate lines cross with a plurality of data lines so as to definelocations for a plurality of pixel cells. FIG. 3 shows a data line 35Lcrossing a gate line 31L and a TFT switching device formed at theintersection. The TFT is constructed such that it overlaps with the gateline 31L, the data line 35L, and with a drain electrode. 35D. The TFTincludes an active layer 33 that is overlapped with the gate line 31L.This reduces leakage current due to exposure by back lighting. A pixelelectrode 37 is then photolithographically formed in the pixel areabetween gate lines and data lines. The pixel electrode electricallyconnects to the drain electrode 35D. Furthermore, the pixel electrodeoverlaps an adjacent gate line 30L, producing a storage capacitor.

According to the principles of the present invention the pixel electrode37 is formed using self-alignment by exposing a transparent conductorlayer that was deposited on the LCD structure by passing light throughthe LCD structure such that the data line, an auxiliary electrode, orthe gate line acts as a mask. By using the data lines and/or the gatelines as masking lines the intervals between the resulting pixelelectrodes and the masking lines becomes uniform.

FIG. 4 illustrates the intervals. As shown, an interval designated “L”on the left of a data line 35L is equal to an interval “R” on the right.Therefore, the image stain defect that arises from uneven intervalsbetween pixel electrodes and data lines is prevented.

Still referring to FIG. 4, auxiliary electrodes 35C comprised of thesame substance as the data lines are included in the LCD. Each auxiliaryelectrode, which electrically contacts a pixel electrode, increases thecapacitance between a gate line 31L and he contacted pixel electrode 37.This increase in capacitance is partially a result of a reduction in thethickness of a dielectric layer between the gate line 31L and the pixelelectrode 37.

FIG. 5A to FIG. 5F show cross-sectional views of the LCD illustrated inFIGS. 3 and 4 during its fabrication. The cross-sectional views aretaken along the II-II′ in FIG. 3.

Referring to FIG. 5A, a first conductive layer is deposited on atransparent substrate 300. The first conductive layer isphotolithographically patterned to form the gate line 31L. A transparentgate insulating layer 32 is then deposited on the gate line 31L and thesubstrate 300. An active layer, which is not shown in FIGS. 5A through5F, is then formed at a predetermined location on the gate insulatinglayer 32.

Referring now to FIG. 5B, a second conductive layer is then depositedover the gate insulating layer 32. An auxiliary electrode 35C and a dataline 35L are then formed using photolithography. Additionally, a drainelectrode that is not shown in the drawing is also formed on the gateinsulating layer 32 at this time.

Referring now to FIG. 5C, a protection layer 36 is then applied to theexposed substrate. A contact hole that exposes a portion of theauxiliary electrode 35C is then photolithography formed through theprotection layer. While not shown in the figures another contact holethat exposes a portion of the drain electrode is also formed. Atransparent conductive layer 37 l is then deposited over the exposedsurface or the substrate. Then, the transparent conductive layer 37 l iscoated with a negative type photoresist layer PR.

Referring to FIG. 5D, exposure of the negative type photoresist layer PRis performed to define photoresist patterns for the pixel electrodes. Asshown in FIG. 5D, the photoresist layer PR is exposured from both sides.This is performed by passing light through the transparent substrate,where the opaque data line 35L and the opaque gate line 31L act asmasks, and from above, where a mask M is used. However, if the pixelelectrodes are not being used to form storage capacitors with the gates,the front side exposure can be skipped. The mask M block lighteverywhere but near the gate layer31L/auxiliary electrode 35C. As shownin FIG. 5D, a small area of the negative type photoresist layer PR isblocked both by the mask M and by the auxiliary electrode 35C.

By exposing the negative type photoresist layer PR through the substrate(referred to as back side exposure) the exposing light exposes thenegative type photoresist layer PR everywhere except where the gate line31L, the data line 35L, and the auxilary electrode 35C (if used) maskthe photoresist layer PR. Furthermore, it should be understood that theexposure steps need not be performed simultaneously. For example,exposure can be carried out by first exposing from the front side andthen from the back, or vice versa.

Referring now to FIG. 5E, the exposed photoresist layer PR is thendeveloped to form a photoresist pattern PR. Referring now to FIG. 5F,pixel electrodes 37 are then formed by etching the transparentconductive layer while using the photoresist pattern PR as a mask. Asshown, each pixel electrode 37 is aligned with a data line 35L sincethat data line acted as a mask during exposure of the negative typephotoresist layer PR. Thus, a uniform interval between the pixelelectrode 37 and the data line 35L is provided and image stains defectsare prevented.

The principles of the present invention address the problem of irregularintervals between data lines and pixel electrodes that result frommisalignment of exposure equipment. This is achieved by using the datalines, gate lines, or auxilary lines as a mask by exposing a photoresistthrough the substrate, back side exposure.

However, the principles of the present invention accomplish even more.For example, they enable the control of the intervals between pixelelectrodes and data lines, gate lines, or auxilary lines. Such controlis explained with the assistance of FIGS. 6A and 6B. For the conveniencethose figures use the same nomenclature as FIGS. 5A through 5B.Referring now to FIG. 6A, back side exposure of the negative typephotoresist pattern PR is carried out through the substrate 300 andthrough the transparent conductive layer 37 l. However, during back-sideexposure the angle of irradiation through the substrate is controlledsuch that the location of the resulting pixel electrode relative to thedata line 35L is controlled. FIG. 6B shows the end result of irradiatingthe data line 35L with light as shown in FIG. 6A. The pixel electrode onthe left side of the data line 35L overlapps with that data line, whilethe pixel electrode on its right side is separated from the data line35L by an interval “R”. One benefit of the structure that results fromFIG. 6B is that light leakage from the left of the data lines isprevented. Of course, light leakage from the right of the data lines canbe prevented by changing the angle of the exposing light. Therefore, theintervals between pixel electrodes 37 and data lines 35L can becontrolled by controlling the direction and angle of light irradiationthrough the substrate.

The principles of the present invention can accomplish even more. Inhigh quality LCD it is very important to control critical dimensions,such as the intervals between pixel electrodes 37 and data lines 35L, bychanging the irradiation angle. For example, the FIG. 7 illustrates adesirable result of controlling the interval 39 within ±1 μm from thepixel electrode 37, using an edge of the data line 35L as a reference.The critical dimension is thereby controlled by photolithography.

Accordingly, the principles of the present invention enables a reduce inthe image stain defect by patterning pixel electrodes such that thepixel electrodes self-align with another element by back-side exposing aphotosensitive layer through a substrate. If a data line is the elementthat is used to self-align the pixel electrodes, a uniform intervalbetween the pixel electrodes and the data lines can result. Moreover,the principles of the present invention enables control of an intervalbetween pixel electrodes and data lines by changing the irradiatingangle through the substrate.

It will be apparent to those skilled in the art that variousmodifications and variation can be made in the present invention withoutdeparting from the spirit or scope of the invention. Thus, it isintended that the present invention covers the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1. A liquid crystal display, comprising: a plurality of gate lines; aplurality of data lines that cross said plurality of gate lines so as todefine a plurality of pixel cells; a plurality of switching devices atintersections between said plurality of gate lines and said plurality ofdata lines; and a plurality of pixel electrodes, wherein each pixelelectrode is in a pixel cell and overlaps a data line, and wherein eachpixel electrode electrically connects to a switching device; wherein adistance between an edge of each pixel electrode is within about 1 μm ofan edge of a data line.
 2. The liquid crystal display according to claim1, wherein two edges of each data line are within about 1 μm of adjacentpixel electrodes.
 3. The liquid crystal display according to claim 1,further including a protection layer disposed between said plurality ofpixel electrodes and said plurality of data lines.
 4. The liquid crystaldisplay according to claim 1, wherein each pixel electrode is separatedfrom an adjacent data line.
 5. The liquid crystal display according toclaim 1, further including an auxiliary electrode that overlaps a firstgate line, wherein said auxiliary electrode is disposed between saidoverlapped first gate line and a first pixel electrode, and wherein saidauxiliary electrode electrically connects to said first pixel electrode.6. A display device, comprising: a plurality of gate lines; a pluralityof data lines crossing the plurality of gate lines to define a pluralityof pixels; a plurality of switching devices near the crossings betweenthe plurality of gate lines and said plurality of data lines; and firstand second pixel electrodes in two of the pixels, wherein one of thedata lines is between the first and second pixel electrodes, the firstpixel electrode overlaps the data line, and an overlap width between thefirst electrode and the data line is substantially the same as adistance between an edge of the data line and an edge of the secondpixel electrode.
 7. The display device according to claim 6, wherein theoverlap width is within about 1 μm.
 8. The display device according toclaim 7, wherein the distance between an edge of the data line and anedge of the second pixel electrode is within about 1 μm.
 9. The displaydevice according to claim 7, wherein the second pixel electrode does notoverlap the data line.